Seasonal and depth-wise variations in bacterial and archaeal groups in the Arabian Sea oxygen minimum zone

Abstract

Seasonal variations, as well as depth-wise distribution patterns of bacterial and archaeal communities in the Arabian Sea oxygen minimum zone (AS-OMZ), were analyzed by examining 16S rRNA gene clones and their sequences. Sampling was carried out at the Arabian Sea Time Series (ASTS) location (17�0.126- N, 67�59.772- E) from five different depths during three different seasons. A total of 743 and 256 non-chimeric bacterial and archaeal 16S rRNA gene sequences were analyzed. Most of the bacterial 16S rRNA gene sequences were affiliated to Gammaproteobacteria (39.31%), Alphaproteobacteria (23.56%) and Cyanobacteria (20.2%). The archaeal 16S rRNA gene sequences mostly aligned with Marine Group II (MG-II, Euryarchaeota). An explicit vertical partitioning of bacterial communities between the surface (surface and DCM) and OMZ (250-m, 500-m, and 1000-m) was observed. Also evident was an apparent seasonal variation among surface bacterial communities but a minimal variation among OMZ bacterial and archaeal communities. LINKTREE and canonical correspondence analysis (CCA) indicates that differences in the concentrations of dissolved oxygen (DO) and total organic carbon (TOC) seem to cause the vertical separation among bacterial communities in the ASTS station. A higher number of shared OTUs affiliated mainly to Alteromonadales (bacteria) and Methanosarcinales (archaea) contributed towards seasonally stable community structure in the OMZ depths.